The present invention relates to machining, and more specifically to an apparatus for drilling or countersinking holes in two workpieces of tubular or annular shape, one nested within the other. Still more specifically, the invention pertains to a variable position work holder mechanism in such an apparatus. The apparatus according to the present invention is used in countersinking (counterboring) holes through which annular reinforcing ribs are to be rivetted to the fuselage of an airplane.
The tubular fuselage of the aircraft needs reinforcement to provide a required degree of mechanical strength. The inside surface of the fuselage is secured to several annular ribs with spacings in the axial direction. Annular rows of rivet holes are formed in the fuselage for fastening the ribs and the fuselage. Therefore, prior to riveting, the ribs are held in position against the inside surface of the fuselage, with the rivet holes in the ribs placed in register with those in the fuselage. Then, the aligned rivet holes are successively countersunk (or counterbored) from outside the fuselage. Thereafter, the ribs are rivetted to the fuselage.
There has been proposed an apparatus known in the present applicant for such countersinking of rivet holes in the nested fuselage and reinforcing ribs (see FIGS. 4 and 5). This conventional apparatus has a drawback which arises when each annular reinforcing rib is made of H-shaped or similar cross section. Each annular reinforcing rib has its outer flange held against the inside surface of a fuselage and is fastened thereto by two annular rows of rivets passing through holes in the outer flange. The two rows of rivet holes in the outer flange of each H-section rib are disposed on both sides of its web.
However, when the fuselage with the H-section ribs placed therein is mounted in position on the known apparatus, the holder mechanism of the apparatus can hold the work for countersinking only one of the two rows of rivet holes in each rib together with the associated row of holes in the fuselage. This is because an anvil assembly of the holder mechanism can be positioned only on one side of the web of each rib. Therefore, according to the prior art, the fuselage with the H-section ribs therein must be remounted with its longitudinal direction reversed, for countersinking the other row of rivet holes in each rib.